Secondary battery deterioration judging device and backup power supply

ABSTRACT

A secondary battery deterioration judging device is provided with an overdischarge detecting section for detecting the overdischarge of a secondary battery, the deterioration of which is to be judged, a computing section for integrating a value indicating the deterioration of the secondary battery while the overdischarge is detected by the overdischarge detecting section, and a judging section for judging a deterioration state of the secondary battery based on an integration value obtained by the computing section.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2007/071775, filed on Nov. 9, 2007,which in turn claims the benefit of Japanese Application No.2006-336834, filed on Dec. 14, 2006, the disclosures of whichApplications are incorporated by reference herein.

FIELD OF TECHNOLOGY

The present invention relates to a secondary battery deteriorationjudging device and a backup power supply.

BACKGROUND TECHNOLOGY

Secondary batteries are widely used as power supplies of mobile devicesand backup power supplies. However, the secondary batteries aredeteriorated by repeated charge and discharge, use at high temperature,overcharge and overdischarge. In the case of such deterioration of asecondary battery, problems such as the shortening of usable time andinability to do a necessary backup occur, wherefore it is important tojudge the deterioration of the secondary battery.

A method for measuring a voltage drop for a current value at the time ofdischarge and calculating internal resistance of the secondary batteryto judge a deteriorated state (see, for example, patent literature 1)and other methods have been proposed as secondary battery deteriorationjudging methods.

However, in the case of a backup power supply or the like, ifdeterioration is judged when the discharge is started as in patentliterature 1, a sufficient discharge capacity cannot be obtained alreadyat that point of time and the occurrence of a problem such as theshortage of a backup time cannot be avoided. Accordingly, measures suchas the implementation of maintenance to perform discharge for regulardeterioration judgments are necessary. Since the power supply cannot bebacked up during the implementation of the maintenance to performdischarge for the deterioration judgment, there are problems such as thenecessity to stop a backup function.

Patent Literature 1:

-   Japanese Unexamined Patent Publication No. 2003-243042

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a secondary batterydeterioration judging device which needs not perform discharge only fora deterioration judgment and a backup power supply using such a judgingdevice.

A deterioration judging device and a backup power supply according toone aspect of the present invention comprises an overdischarge detectingsection for detecting the overdischarge of a secondary battery, thedeterioration of which is to be judged; a computing section forintegrating a value indicating the deterioration of the secondarybattery while the overdischarge is detected by the overdischargedetecting section; and a judging section for judging a deteriorationstate of the secondary battery based on an integration value obtained bythe computing section.

According to this construction, the overdischarge of the secondarybattery, the deterioration of which is to be judged, is detected by theoverdischarge detecting section. Further, the value indicating thedeterioration of the secondary battery is integrated while theoverdischarge of the secondary battery is detected by the overdischargedetecting section. Since the deterioration state of the secondarybattery is judged by the judging section based on the integration valueobtained by the computing section, the deterioration of the secondarybattery can be judged without performing any discharge only for thedeterioration judgment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic construction diagram of a system according to oneembodiment of the invention,

FIG. 2 is a graph showing a plurality of overcharge judgment levels anda voltage state of a secondary battery,

FIG. 3 is a graph showing a relationship between a charge/dischargecycle number and a discharge capacity of the secondary battery.

BEST MODES FOR EMBODYING THE INVENTION

Hereinafter, the principle of a secondary battery deterioration judgmentaccording to the present invention is described before one embodiment ofthe present invention is specifically described.

Generally, deteriorations of a secondary battery are mainly caused by adecrease of an electrolyte by electrolysis and the passivation of activesubstances due to overcharge and also by a decrease of the electrolyteby electrolysis and the passivation and inactivation of activesubstances due to overdischarge in many cases.

As for the deterioration caused by overcharge out of thesedeteriorations, there is little likelihood of inducing a suddendeterioration since the secondary battery is designed to be resistant toovercharge more or less by optimizing a balance design of a positiveelectrode capacity and a negative electrode capacity. On the other hand,as for the deterioration caused by overdischarge, a degree of thedeterioration is considerably larger than the one caused by overchargesince an electrochemical reaction totally different from the one in anormal state such as the reversal of a potential relationship of apositive electrode and a negative electrode at the time of overdischargeis induced.

A secondary battery used in a backup power supply has tended to have alarger size and a higher voltage with the development of informatizationin recent years. Particularly, in the case of large power consumption,an increase in Joule heat generation of circuits and elements becomesproblematic if an attempt is made to increase an output current. Thus,output power is generally increased while an increase of the outputcurrent is suppressed by increasing an output voltage.

In this case, the number of cells connected in series increases sincethe cell voltage of the secondary battery is electrochemicallydetermined. However, if the cells are connected in series, a currentflowing through all the cells connected in series is constant, whereforea cell having a low SOC (state of charge) is overdischarged at a finalstage of discharge due to differences in the SOC among the cells.

Accordingly, overdischarge needs to be avoided by overdischargedetecting means. However, if discharge is stopped upon the overdischargeof one cell, the discharge of the cells with remaining capacities isrestricted even if the other cells have remaining capacities and havesufficient backup abilities. Therefore, there is a problem ofrestricting the ability of the entire power supply.

Thus, the prevent invention aims to enable a deterioration judgment soas to implement necessary battery maintenance at a suitable timing byprecisely grasping the deterioration of a secondary battery caused byoverdischarge while the ability of the entire power supply is maximallyexhibited when a backup is necessary.

FIG. 3 is a graph showing an experimental result on a comparison betweena case where a durability test is conducted while a secondary battery isoverdischarged and a case where a durability test is conducted withoutthe secondary battery being overdischarged. Batteries used in thedurability tests are cylindrical nickel-hydrogen storage batteries of3000 mAh. In ambient temperature 20° C., these batteries were fullycharged by being charged until a battery voltage dropped by 10 mV from apeak value with a charge current set to 3 A, and were further dischargedat a discharge current of 10 A until a terminal voltage dropped to 0 V.

The discharge of the secondary batteries not to be overdischarged wasstopped here. The test was conducted for two types of secondarybatteries to be overdischarged: those that were, thereafter,overdischarged for 20 sec. at 10 A and those that were overdischargedfor 40 sec. Cells which were not overdischarged have capacities of 98%or higher of the initial capacity even after 100 cycles and were hardlydeteriorated.

On the other hand, as for cells overdischarged at 10 A for 20 sec. inevery discharge cycle, the discharge capacity dropped to 95% of theinitial capacity in the 36^(th) Cycle and to 90% of the initial capacityin the 68^(th) cycle. As for cells overdischarged at 10 A for 40 sec. ineach discharge cycle, the discharge capacity dropped to 95% of theinitial capacity in the 20^(th) cycle and to 90% of the initial capacityin the 36^(th) cycle.

It is clear from this result that the overdischarge of the batterycauses a sudden promotion of capacity deterioration. Further, since thecells overdischarged at 10 A for 40 sec. were deteriorated substantiallytwice as fast as those overdischarged at 1 OA for 20 sec., it is alsoclear that a good correlation can be found between the overdischargeamount and the deterioration.

Since the very good correlation is found between the overdischargeamount and the deterioration of the battery in this way, thedeterioration of a secondary battery can be precisely grasped bydetecting the overdischarge of the secondary battery and integratingtime during which the overdischarge continues in such a case where aload current is virtually supposed. Further, if a variation of the loadcurrent is large and it is difficult to suppose a current value, thedeterioration of the secondary battery can be precisely grasped byintegrating a discharged electric amount obtained from the current atthe time of discharge and an overdischarge time as an overdischargeduration.

Further, depending on the type of the battery, a discharge level maychange stepwise and the cause of promoting the deterioration may differat each level. In such a case, the deterioration of the secondarybattery can be precisely judged by providing a plurality of judgmentlevels in overdischarge detecting means and integrating theoverdischarge times and integrated overdischarge electric amounts judgedat the respective judgment levels using different coefficients.

Hereinafter, best modes for embodying the present invention aredescribed with reference to the drawings.

First Embodiment

FIG. 1 is a schematic construction diagram of a backup power supplysystem using a deterioration judging device according to a firstembodiment of the present invention.

The backup power supply system shown in FIG. 1 includes a battery module1, a current measuring section 4 and an ECU (Electric Control Unit) 5.In this case, an example of the deterioration judging device isconstructed by the current measuring section 4 and the ECU 5.

The battery module 1 is connected in series with the current measuringsection 4 and is also connected to a load. The battery module 1 isconstructed by connecting at least one or more secondary batteries 2 inseries. A voltage of the secondary battery 2 is connected with an inputof an overdischarge detecting section 3 provided in the ECU 5.

The current measuring section 4 is connected to an overdischargeelectric amount integrating section 7 in the ECU 5. A memory 8 and a CPU9 (judging section) are further included in the ECU 5. The CPU 9 isconnected to the memory 8, the overdischarge detecting section 3, theoverdischarge electric amount integrating section 7, and anoverdischarge time integrating section 6 in the ECU 5 to transmit andreceive data and control the respective parts.

The overdischarge detecting section 3 measures voltages of therespective secondary batteries 2 and sends data of the voltage values tothe CPU 9. The CPU 9 judges an overdischarge level based on the receivedvoltage value data. The current measuring section 4 measures a dischargecurrent or charge current of the battery module 1 and sends data of thecharge/discharge current value to the overdischarge electric amountintegrating section 7.

Upon discharge from the battery module 1 to the load, the voltage of thesecondary battery 2 drops and consequently drops below an overdischargevoltage value V1 set beforehand. Then, the overdischarge detectingsection 3 detects an overdischarge state and notifies it to the CPU 9.The CPU 9 causes the overdischarge time integrating section 6 tointegrate the duration of the overdischarge state. In other words, theoverdischarge time integrating section 6 (overdischarge time measuringsection) measures a time during which the voltage of the secondarybattery 2 is below the overdischarge current value V1 and assumes thisintegrated duration to be T1. Further, the overdischarge timeintegrating section 6 calculates A×T1, which is a value proportional tothe integrated duration T1, as an integration value.

The CPU 9 uses A×T1, which is the value calculated by the overdischargetime integrating section 6 and proportional to the integrated durationT1, for the secondary battery deterioration judgment. Here, A is acoefficient indicating a deterioration degree when the voltage of thesecondary battery 2 is equal to or below V1.

FIG. 3 shows a relationship between a battery capacity and anoverdischarge time. As can be understood from this result, since thebattery capacity is linearly related to a charge/discharge cycle number,i.e. the overdischarge time, the deterioration of the secondary battery2 can be judged if the integration value A×T1 exceeds a predeterminedvalue Z1. The CPU 9 judges whether or not the battery has beendeteriorated by comparing the integration value A×T1 with the value Z1stored in the memory 8.

Specifically, the CPU 9 judges the deterioration of the secondarybattery 2 if the integration value A×T1 becomes equal to or larger thanZ1 while judging no deterioration of the secondary battery 2 if theintegration value A×T1 is below Z1.

Since the backup power supply shown in FIG. 1 judges the deteriorationstate of each secondary battery 2 from the integrated amount of theoverdischarge time of the secondary battery 2 in this way, thedeterioration of the secondary battery can be judged withoutimplementing any maintenance to perform discharge for the deteriorationjudgment by stopping the backup function. Thus, a situation where thesecondary battery is already deteriorated and a sufficient backup timecannot be obtained upon actually doing a backup between regularlyimplemented maintenances can be avoided beforehand.

When the secondary battery 2 is charged and the voltage exceeds V1 toexit from the overdischarge state, the CPU 9 causes the overdischargetime integrating section 6 to stop the integration operation and causesthe duration T1 integrated by the overdischarge time integrating section6 to be stored in the memory 8. The overdischarge time integratingsection 6 starts integrating from the integrated duration T1 stored inthe memory 8 upon restarting the integration of the overdischarge time.

Second Embodiment

There are a plurality of overdischarge levels (degrees) of a secondarybattery 2 according to the battery voltage of the secondary battery 2.FIG. 2 is a graph showing a plurality of overdischarge judgment levelsand a state of the battery voltage of the secondary battery 2. In FIG.2, two overdischarge levels are shown. When the secondary battery 2 isdischarged, a positive electrode of the battery first enters anoverdischarge state. A region X shown in FIG. 2 indicates this state.

When the discharge is further continued from this state and the batteryvoltage of the secondary battery 2 drops, transition is made to a regionwhere both positive and negative electrodes are in the overdischargestate (Y of FIG. 2). It is assumed that the battery voltage is betweenV1 (inclusive) and V2 (inclusive) in the region X and below V2 in theregion Y. In the respective states, a degree of influence on thedeterioration of the battery differs. Accordingly, it is preferable toprovide a plurality of coefficients for determining a deteriorationdegree and measure the duration in each overdischarge state.

Specifically, if T1 and T2 denote an integrated duration during whichthe battery voltage is between V1 (inclusive) and V2 (inclusive) and anintegrated duration during which the battery voltage is below V2, thedeterioration of the battery can be judged if the sum A×T1+B×T2 of theintegration values at the respective overdischarge levels exceeds apredetermined value Z2. Here, A and B are respectively coefficientsindicating a deterioration degree (degree of deterioration) when thebattery voltage is between V1 (inclusive) and V2 (inclusive) and adeterioration degree (degree of deterioration) when the battery voltageis below V2, and A<B.

In this case, the deterioration of the secondary battery 2 can bejudged, considering the degree of influence on the deterioration of thebattery depending on the degree of overdischarge, wherefore the accuracyof the deterioration judgment can be improved.

Although there are two overdischarge levels here, the number of setlevels is not limited thereto and integrating operations may beperformed while setting three or more levels.

Third Embodiment

The overdischarge deterioration of a secondary battery also depends on adischarge current in an overdischarge state. If the voltage of thesecondary battery 2 drops to or below a predetermined value V1 and theoverdischarge detecting section 3 detects the overdischarge of thesecondary battery 2, the overdischarge detecting section 3 notifies itto the CPU 9 and the overdischarge electric amount integrating section 7starts integrating the discharge electric amount using data of a batterymodule discharge current I sent from the current measuring section 4 inaccordance with an instruction of the CPU 9.

If the discharge current I is constant, C×I×T1 proportional to a productof the overdischarge time T1 and the current I serves as an integrationvalue of the overdischarge electric amount and indicates thedeterioration degree of the battery. The deterioration of the battery isjudged when the integration value of the overdischarge electric amountobtained in this way exceeds a predetermined value Z3. Here, C is acoefficient indicating a deterioration degree (degree of deterioration)when the voltage of the secondary battery is equal to or below V1.

Since the discharge current is not always constant, the currentmeasuring section 4 measures the current in each predetermined cycle. IfΔt denotes a measurement cycle, N the number of current measurementsmade during the overdischarge duration, I(k) a k^(th) currentmeasurement value and Z0 a deterioration degree before the overdischargedetection this time, a new integration value is calculated to beZ0+C×(I(1)+I(2)+ . . . +I(N−1)+I(N))×Δt by the overdischarge electricamount integrating section 7.

The CPU 9 judges the deterioration of the battery when the newintegration value obtained in this way exceeds the predetermined valueZ3.

Fourth Embodiment

The deterioration of a battery is influenced by an overdischarge statewhich can be judged form a battery voltage, a discharge current at thattime and the duration of the overdischarge state. Depending on anoperation condition of a system, even if some of batteries constitutinga battery system (backup power supply) enter an overdischarge state, butthe other batteries are not yet overdischarged, there are cases wherethe system preferably rather continues to be operated without beingstopped.

Now, if it is assumed that T1 denotes a duration until the voltage of asecondary battery drops from V1 to V2 after dropping to V1, T2 aduration during which the voltage is below V, and I a discharge current,the deterioration of the secondary battery at this time is calculated tobe C×I×T1+D×I×T2 by the overdischarge electric amount integratingsection 7.

The CPU 9 judges the deterioration of the battery if the valueC×I×T1+D×I×T2 calculated by the overdischarge electric amountintegrating section 7 exceeds a predetermined value Z3. Here, C and Dare respectively coefficients indicating a deterioration degree (degreeof deterioration) until the battery voltage of the secondary batterydrops from V1 to V2 and a deterioration degree (degree of deterioration)when the battery voltage of the secondary battery is below V2, and C<D.

As also described in the third embodiment, the discharge current is notalways constant. If Δt denotes a current measurement cycle by thecurrent measuring section 4, N the number of current measurements madein an overdischarge state where the voltage of the secondary battery isbetween V1 and V2, M the number of current measurements made in anoverdischarge state where the voltage of the secondary battery is belowV2, I(k) a k^(th) current measurement value and Z0 a deteriorationdegree before the overdischarge detection this time, a new integrationvalue is calculated to be Z0+C×(I(1)+I(2)+ . . .+I(N−1)+I(N))×Δt+D×(I(1)+I(2)+ . . . +I(M−1)+I(M))×Δt by theoverdischarge electric amount integrating section 7.

The CPU 9 judges the deterioration of the battery when the newintegration value calculated by the overdischarge electric amountintegrating section 7 exceeds the predetermined value Z3.

A computing section as claimed is equivalent to the overdischarge timeintegrating section 6, the overdischarge electric amount integratingsection 7 or a combination of the overdischarge time integrating section6 and the overdischarge electric amount integrating section 7.

A deterioration judging device according to one aspect of the presentinvention comprises an overdischarge detecting section for detecting theoverdischarge of a secondary battery, the deterioration of which is tobe judged; a computing section for integrating a value indicating thedeterioration of the secondary battery while the overdischarge isdetected by the overdischarge detecting section; and a judging sectionfor judging a deterioration state of the secondary battery based on anintegration value obtained by the computing section.

According to this construction, the overdischarge of the secondarybattery, the deterioration of which is to be judged, is detected by theoverdischarge detecting section. Further, the value indicating thedeterioration of the secondary battery is integrated while theoverdischarge of the secondary battery is detected by the overdischargedetecting section. Since the deterioration state of the secondarybattery is judged by the judging section based on the integration valueobtained by the computing section, the deterioration of the secondarybattery can be judged without performing any discharge only for thedeterioration judgment.

It is preferable that an overdischarge time measuring section formeasuring a period during which the overdischarge is detected by theoverdischarge detecting section as an overdischarge time is furtherprovided; and that the computing section integrates the overdischargetime measured by the overdischarge time measuring section as the valueindicating the deterioration of the secondary battery.

According to this construction, the period during which theoverdischarge is detected by the overdischarge detecting section ismeasured as the overdischarge time by the overdischarge time measuringsection. The overdischarge time measured by the overdischarge timemeasuring section is integrated as the value indicating thedeterioration of the secondary battery by the computing section. Sincethe overdischarge time is correlated with the deterioration of thesecondary battery, the integration value of the overdischarge time ispreferable as the value indicating the deterioration of the secondarybattery.

It is preferable that a judgment level used to judge the degree ofoverdischarge is set beforehand; that the overdischarge detectingsection detects the degree of overdischarge based on the judgment level;that the computing section obtains the value indicating thedeterioration of the secondary battery by multiplying the overdischargetime measured by the overdischarge time measuring section by acoefficient indicating the degree of overdischarge detected by theoverdischarge detecting section; and that the coefficient is set toincrease as the degree of overdischarge increases.

According to this construction, the degree of overdischarge is detectedby the overdischarge detecting section based on the judgment level usedto judge the degree of overdischarge. Further, the computing sectionobtains the value indicating the deterioration of the secondary batteryby multiplying the overdischarge time by the coefficient set to increaseas the degree of overdischarge detected by the overdischarge detectingsection increases. In this case, since the overdischarge time isintegrated as the value indicating the deterioration of the secondarybattery in consideration of the degree of overdischarge, accuracy injudging the deterioration of the secondary battery is improved.

A current measuring section for measuring the value of a current flowingin the secondary battery may be further provided, and the computingsection may integrate the current value measured by the currentmeasuring section while the overdischarge is detected by theoverdischarge detecting section as the value indicating thedeterioration of the secondary battery.

According to this construction, the value of the current flowing in thesecondary battery is measured by the current measuring section. Further,the current value measured by the current measuring section while theoverdischarge is detected by the overdischarge detecting section isintegrated as the value indicating the deterioration of the secondarybattery by the computing section. Since the current value in theoverdischarge state is correlated with the deterioration of thesecondary battery, the integration value of such a current value ispreferable as the value indicating the deterioration of the secondarybattery.

It is preferable that a judgment level used to judge the degree ofoverdischarge is set beforehand; that the overdischarge detectingsection detects the degree of overdischarge based on the judgment level;that the computing section obtains the value indicating thedeterioration of the secondary battery by multiplying the current valuemeasured by the current measuring section while the overdischarge isdetected by the overdischarge detecting section by a coefficientindicating the degree of overdischarge detected by the overdischargedetecting section; and that the coefficient is set to increase as thedegree of overdischarge increases.

According to this construction, the degree of overdischarge is detectedby the overdischarge detecting section based on the judgment level usedto judge the degree of overdischarge. Further, the computing sectionobtains the value indicating the deterioration of the secondary batteryby multiplying the current value measured by the current measuringsection while the overdischarge is detected by the overdischargedetecting section by the coefficient set to increase as the degree ofoverdischarge of the secondary battery increases. In this case, sincethe value of the current flowing in the secondary battery is integratedas the value indicating the deterioration of the secondary battery inconsideration of the degree of overdischarge, accuracy in judging thedeterioration of the secondary battery is improved.

The overdischarge detecting section preferably detects the overdischargeof the secondary battery when a terminal voltage of the secondarybattery drops below an overdischarge voltage value set beforehand.

Since the terminal voltage of the secondary battery decreases as thedischarge proceeds, the construction of the overdischarge detectingsection can be easily simplified by detecting the overdischarge of thesecondary battery based on whether or not the terminal voltage of thesecondary battery is below the overdischarge voltage value setbeforehand.

It is preferable that the judgment level is a voltage value set todecrease as the degree of overdischarge increases; and that theoverdischarge detecting section detects the degree of overdischargebased on a comparison result of the terminal voltage of the secondarybattery and the judgment level.

Since the terminal voltage of the secondary battery decreases as thedischarge proceeds, the overdischarge detecting section can easilydetect the degree of overdischarge based on the comparison result of theterminal voltage of the secondary battery and the judgment level bysetting the voltage value set to decrease as the degree of overdischargeincreases as the judgment level.

Further, a backup power supply according to one aspect of the presentinvention comprises a secondary battery; an overdischarge detectingsection for detecting the overdischarge of the secondary battery; acomputing section for integrating a value indicating the deteriorationof the secondary battery while the overdischarge is detected by theoverdischarge detecting section; and a judging section for judging adeterioration state of the secondary battery based on the integrationvalue obtained by the computing section.

According to this construction, the overdischarge of the secondarybattery is detected by the overdischarge detecting section. Further,while the overdischarge of the secondary battery is detected by theoverdischarge detecting section, the value indicating the deteriorationof the secondary battery is integrated by the computing section. Sincethe deterioration state of the secondary battery is judged based on theintegration value obtained by the computing section, the deteriorationof the secondary battery can be judged without performing any dischargeonly for a deterioration judgment.

INDUSTRIAL APPLICABILITY

The present invention is useful for a secondary battery deteriorationjudgment and has a profound effect particularly when being used in abackup power supply.

What is claimed is:
 1. A secondary battery deterioration judging device,comprising: an overdischarge detecting section for detecting a terminalvoltage of a secondary battery of which a deterioration is to be judged,determining whether the detected terminal voltage is equal to or lessthan a predetermined voltage value, and detecting an overdischarge stateof the secondary battery when the detected terminal voltage is equal toor less than the predetermined voltage value; a computing section forintegrating a value indicating the deterioration of the secondarybattery while the overdischarge state is detected by the overdischargedetecting section; and a judging section for judging a deteriorationstate of the secondary battery based on an integration value obtained bythe computing section, wherein the overdischarge detecting sectionincludes, as the predetermined voltage value, a first voltage valuewhich is a positive value or zero and a second voltage value which isless than zero, and detects a first overdischarge state of the secondarybattery when the detected terminal voltage is equal to or less than thefirst voltage value and equal to or more than the second voltage value,and detects a second overdischarge state of the secondary battery whenthe detected terminal voltage is less than the second voltage value. 2.A secondary battery deterioration judging device according to claim 1,further comprising an overdischarge time measuring section for measuringa period during which the first overdischarge state is detected by theoverdischarge detecting section as a first overdischarge time and aperiod during which the second overdischarge state is detected by theoverdischarge detecting section as a second overdischarge time, whereinthe computing section integrates the first overdischarge time measuredby the overdischarge time measuring section as a first value indicatingthe deterioration of the secondary battery, and integrates the secondoverdischarge measured by the overdischarge time measuring section as asecond value indicating the deterioration of the secondary battery, andthe judging section judges the deterioration state of the secondarybattery based on the integration value of the first value and theintegration value of the second value obtained by the computing section.3. A secondary battery deterioration judging device according to claim2, wherein: the computing section obtains the integration value of thefirst value indicating the deterioration of the secondary battery bymultiplying the first overdischarge time measured by the overdischargetime measuring section by a first coefficient corresponding to the firstoverdischarge state detected by the overdischarge detecting section, andobtains the integration value of the second value indicating thedeterioration of the secondary battery by multiplying the secondoverdischarge time measured by the overdischarge time measuring sectionby a second coefficient corresponding to the second overdischarge statedetected by the overdischarge detecting section; and the secondcoefficient is set to a value greater than the first coefficient.
 4. Asecondary battery deterioration judging device according to claim 1,further comprising a current measuring section for measuring the valueof a current flowing in the secondary battery, wherein the computingsection integrates the current value measured by the current measuringsection while the first overdischarge state is detected by theoverdischarge detecting section as a first value indicating thedeterioration of the secondary battery, and integrates the current valuemeasured by the current measuring section while the second overdischargestate is detected by the overdischarge detecting section as a secondvalue indicating the deterioration of the secondary battery, and thejudging section judges the deterioration state of the secondary batterybased on the integration value of the first value and the integrationvalue of the second value obtained by the computing section.
 5. Asecondary battery deterioration judging device according to claim 4,wherein: the computing section obtains the integration value of thefirst value indicating the deterioration of the secondary battery bymultiplying the current value measured by the current measuring sectionwhile the first overdischarge state is detected by the overdischargedetecting section by a first coefficient corresponding to the firstoverdischarge state detected by the overdischarge detecting section, andobtains the integration value of the second value indicating thedeterioration of the secondary battery by multiplying the current valuemeasured by the current measuring section while the second overdischargestate is detected by the overdischarge detecting section by a secondcoefficient corresponding to the second overdischarge state detected bythe overdischarge detecting section; and the second coefficient is setto a value greater than the first coefficient.
 6. A backup power supply,comprising: a secondary battery; an overdischarge detecting section fordetecting a terminal voltage of the secondary battery, determiningwhether the detected terminal voltage is equal to or less than apredetermined voltage value, and detecting an overdischarge state of thesecondary battery when the detected terminal voltage is equal to or lessthan the predetermined voltage value; a computing section forintegrating a value indicating the deterioration of the secondarybattery while the overdischarge state is detected by the overdischargedetecting section; and a judging section for judging a deteriorationstate of the secondary battery based on the integration value obtainedby the computing section, wherein the overdischarge detecting sectionincludes as the predetermined voltage value, a first voltage value whichis a positive value or zero and a second voltage value which is lessthan zero, and detects a first overdischarge state of the secondarybattery when the detected terminal voltage is equal to or less than thefirst voltage value and equal to or more than the second voltage value,and detects a second overdischarge state of the secondary battery whenthe detected terminal voltage is less than the second voltage value.